#bag {
    width: 600px;
    height: 300px;
    margin: 0 auto;
    background: url(img/before.jpg) center no-repeat;
    background-size: 80%;
}

#bag.burst {
    background-image: url(img/after.jpg);
}

#bag-health{
    width: 478px;
    border: 2px #000 solid;
    margin: 0 auto 20px auto;
}

#bag-health div {
    height: 20px ;
    background: crimson;
}

#controls {
    width: 180px;
    margin: 0 auto;
}

#controls button {
    margin-left: 20px;
}